Attenuation Flashcards
What is attenuation?
The weakening of the sound beam as it travels
Is attenuation good or bad? Why is it important?
It is both pos and neg.
1) Limits our imaging depth (neg)
2) Must be compensated for (neg)
3) Can be useful in diagnostics (pos)
What is used to compensate for attenuation?
TGC’s and 2D Gain
What does TGC’s stand for?
Time Gain Adding Compensation
How is attenuation useful in diagnosis?
An increase in attenuation (shadow) behind a mass indicates that the mass is solid or stone. A cystic mass (fluid filled) would show a lack of attenuation (acoustic enhancement/through transmission/brightness) behind the structure.
What are the relative units of attenuation?
bel (B) and the decibel (dB)
What do bels and decibels express?
The loudness (intensity level) of soundwaves
1 bel = ?
10 decibels
How is the power of the sound leaving the transducer determined?
Instrument output using decibels
What is dynamic range?
A button on the machine controlling the amount of contrast.
More shades of grey & less contrast vs. Fewer shades of grey and more contrast.
How does dynamic range work?
Dynamic range uses decibels to express the number of shades of grey displayed on the monitor.
How do gain and TGC’s work?
They use decibels to express the amount of amplification required to optimize the returning echos.
What is the formula to calculate bels?
Bel = log(New I/original I)
What is the formula to calculate decibels?
dB = 10log(new I/original I)
What is the formula to calculate power?
dB = 10 log(new p/original p)
What is the formula to calculate voltage?
dB = 20log(new v/original v)
What is the trick for remembering the formulas for calculating intensity(bels + decibels), power and voltage?
Same formula but:
Bel = log
Decibel = 10log
Power = 10log
Voltage = 20log
What is the “rule of thumb” when dealing with decibels?
A 3dB drop = 1/2 the original intensity
A 10 dB drop = 0.1 of the original intensity
In a question calculating attenuation, is the “output” value the ‘new’ or ‘original’ value?
Output = New
Input = Original
Practice Intensity/power/voltage calculations using logs
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What is an ‘attenuation coefficient’?
The amount of attenuation that occurs with one centimeter travelled.
What is the amount of attenuation in soft tissue for every centimeter travelled per 1 MHz?
0.5 dB per 1 MHz
What is the attenuation coefficient in soft tissue?
1/2 frequency
What is the formula for total attenuation?
TA = Attenuation Coefficient x path length (cm)
What is the formula for total attenuation in soft tissue?
TA = 1/2f x Path Length (cm)
When doing calculations for “back at the probe” what do we need to be careful of?
Depth
“Back at the probe” means the total trip there and back, if the depth into the soft tissue is given it must be doubled in the equation to account for the trip there and back.
When doing calculations where we are plugging in a found value for total attenuation (in decibels) what must we remember?
That the total attenuation value will be a NEGATIVE number, because attenuation is a loss.
To get rid of a log on one side of the equation?
Anti log the other side
2nd Function Log
What is half-value layer?
The distance sound must travel in a material to reduce intensity to half it’s original value.
What is half the original intensity equal to in decibels?
3 dB loss
What is the half value layer for air? (At 4 diff frequencies)
1 MHz = .25
2 MHz = .06
5 MHz = 0.01
10 MHz = 0
What 3 factors vary attenuation?
1) Nature of the tissue (Dead or alive)
2) Frequency of the ultrasound
3) Depth
What are the 5 reason attenuation occurs?
1) Absorption
2) Reflection
3) Refraction
4) Scatter
5) Wave-front Divergence
What is absorption?
The conversion of sound energy to heat (vibration from friction)
What is the dominant factor in attenuation?
Absorption
What is the number one reason of attenuation and what is the percentage?
Absorption accounts for 80% of attenuation.
What are the three factors effecting absorption?
1) Viscosity
2) Relaxation time on molecules
3) Frequency
What is Viscosity?
The easy in which molecules can slide past eachother (how well a liquid pours).
What happens with increased viscosity?
Increased viscosity means greater resistance which creates more friction and therefore increased attenuation.
What is friction?
How sound energy is converted to heat.
What happens when a mechanical force is applied to a molecule?
It vibrates
What is relaxation time?
When a mechanical force is applied to a molecule it will vibrate, the time is takes for the molecule to come to rest is called “relaxation time”.
What happens if a molecule doesn’t come to rest before the next compression phase?
More energy is required to reverse it’s direction, producing more heat.
Ex. Think of pushing a person on a swing as they are swinging towards you Vs. If they are not moving.
How does relaxation time vary in soft tissue?
It doesn’t much, it’s relatively constant.
What influences absorption more, relaxation time or frequency? Why?
Relaxation time in soft tissue is relatively constant so frequency has more influence.
What happens to absorption when frequency is increased?
There is less time available for molecules to recover during the relaxation process so more absorption occurs.
An increase in what three things increases absorption?
Increase in frequency
Increase in viscosity
Increase in relaxation
All equal more absorption.
How many types of reflection are there and what are they?
Two
1) Specular
2) Non-specular
What is specular reflection?
Occurs when a beam hits a large, smooth surface
What is considered a “large surface” for specular reflection to occur? (3 factors)
1) Relative to the frequency used
2) Greater than one wavelength in diameter
3) Anything over .3 mm
What reason for attenuation contributes to the majority of our image?
Reflection from interfaces
What is an example of a good specular reflector?
The diaphragm
What does the intensity of reflected sound depend on?
1) Angle of incidence
2) Acoustic impedance of two media
What is the “angle of incidence”?
The angle the sound beam hits the reflector, perpendicular or non-perpendicular.
What does the angle of incidence equal?
The angle of reflection
What happens when a reflection of sound has a non-perpendicular angle of incidence?
Sound may not return back to the probe.
What helps improve reflection?
Perpendicular incidence.
How fast sound moves through a medium depends on what?
Density and Stiffness
As density decreases or stiffness increases, what happens to propagation speed?
It increases
What is another name for acoustic impedance?
Characteristic impedance
What is acoustic impedance?
The relationship between acoustic pressure and the speed of particle vibrations in a sound wave (speed propagation).
What is the formula for acoustic impedance?
Impedance (Z) = density x velocity
In short, what is acoustic impedance?
The Z value
What is the unit of acoustic impedance?
Rayls (Z)
When density or velocity increase, what happens to the Z value?
It increases
How Z values vary?
In different tissues since there are variations in stiffness and density throughout the body.
If you change frequency, how is the Z value effected?
It doesn’t change. Acoustic impedance does not depend on frequency.
How is the size of the reflection effected in regards to Z values?
The larger the Z value difference at the interface of two media, the bigger the reflection.
What is the intensity reflection coefficient?
The calculation of the amount of sound that reflects at an interface (how much sound comes back).
What is the Intensity Reflection Coefficient formula?
IRC = Reflected Intensity (Ir) / Incident Intensity (Ii)
OR
IRC = (Z2 - Z1/Z2 + Z1)^2 x 100%
What is the term for the amount of sound transmitted?
Intensity transmission coefficient
What is the formula for Intensity Transmission Coefficient?
ITC = 1 - IRC
What is the relationship between sound transmitted and sound reflected?
Inverse
The more sound reflected the less sound transmitted
As the mismatch in impedance between 2 tissues across an interface increases, how is the sound effected?
As the mismatch in impedance increases, the more sound will be reflected.
What happens if the impedance is equal across the interface?
No reflection will occur.
What is the unit of IRC (Intensity Reflection Coefficient)?
%
What is the amount of reflected sound at an air/soft tissue interface?
Why is this important in UT?
IRC = 99.9%
(Only 0.1% comes back to the probe)
Important because this is why we need gel.
In short, why does reflection occur?
A difference in Z values across an interface.
Is it possible for 2 tissues to have different sound velocities but have the same Z values?
Yes
What is refraction?
The sound bending across and interface (what happens when the velocities differ and the angle is non-perpindicular across an interface).
What is Snell’s law of optics?
Sound can bend across an interface due to a difference in media velocity.
(Sin θi/Sin θt) = vi/vt
θi = Sound in
θt = Sound out
What do we need to know in order to determine how much sound will bend?
1) The velocity of the tissues across the interface
2) The angle the sound comes in at
What happens to refraction if the velocities across the interface are equal?
No refraction will occur.
If the velocity of the first medium is greater that the second, what happens to the sound?
The sound will bend towards the normal
V1 > V2
If the velocity of the first medium is less than the second, what happens to the sound?
The sound will bend away from the normal.
V1 < V2
What happens when V1 is less than V2 and the sound reaches a critical value?
Total Internal Reflection - No image is produced, sound travels along the interface.
Only happens when V2 > V1
In short, why does refraction occur?
Difference in velocities across an interface coupled with a non-perpendicular angle of incidence.
Does refraction have anything to do with Z values?
No
What is scatter?
Scatter is the result of sound interacting with rough interfaces that are smaller than a wavelength.
What is heterogenous media?
Cells or suspended particles
What reason for attenuation is responsible for internal texture of organs?
Scatter
Describe what things cause scatter and their relation to sound direction?
Scatter usually from heterogenous media and is independent of sound direction.
What type of reflection is scatter?
Non-specular
What is parenchyma?
Internal structure of organs
What is the result of scatter?
The incident sound beam breaks into many different echos, these echos have intensities that are a fraction of the incident beam and they reflect in unpredictable directions.
What does the amount of scatter depend on?
1) Frequency
2) Reflector size
How does frequency effect scatter?
The higher the frequency, the greater the amount of scatter.
How does reflector size effect scatter?
The smaller the reflector, the greater the amount of scatter.
What is backscatter?
When sound is directed back to it’s origin.
What type of scatter is responsible for the parenchyma we see on UT?
Backscatter.
What is brightness non-uniformities?
Back scatter is random and is the result of interference patterns from echos that have undergone multi-path scattering.
What is acoustic speckle?
Bright and dark spots created from multipath back scatter.
What is the specific type of scatter that results from sound interacting with red blood cells?
Rayleigh scatter
What are the dimentions of a red blood cell?
Much smaller than a wavelength of sound
Describe the character of rayleigh scatter?
It is very weak and is the reason why we do not see blood flow at faster velocities.
Why does rayleigh scatter happen?
Red blood cells slow down and clump together
What does rayleigh scatter look like on UT?
Smoke in blood vessels
What is wave front divergence?
As a sound beam diverges over an area the intensity of the beam weakens and adds to the overall attenuation of the beam.
(Area increases, intensity decreases so we are losing sound).
Go over all example equations in attenuation notes.
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